I remember reading that *some* of the leftover code was still in Gen II from where they planned to include following Pokémon, but not sure how much if any is there or how usable it is. Off the top of my head, I remember that when referring to events in movement 0x00 is the player, 0x01 is unused, and 0x02 is where people events on the map start, and to me that indicates that 0x01 was at least originally reserved for the Pokémon following you, although at this point it is probably just unused.

But yeah, full on yellow-style follow me scripts would/will be a mess and people haven't even fully pulled it off in Gen III yet either. But I remember the girl you put in DE and I remember reading tutorials on doing similar things in RPG Maker where you had events that represent the other party members following you. It involved having a copy of those placeholder events on every map and using map scripts to make them teleport to you when you entered the map, which would be super annoying to try and do here.